1. Field of the Invention
The present invention relates to an electrical control circuit for generating signals to operate a circuit interrupter in a line carrying alternating current and, more specifically, to a circuit for generating a trip signal in accordance with a predetermined time-current characteristic and for initiating the interruption of overcurrent at a time when the instantaneous current is within the interrupting capability of the circuit interrupter at the time of interruption for overcurrents which have instantaneous values greater than the interrupting capability of the circuit interrupter.
2. Description of the Related Art
Various control circuits are known in the prior art for controlling circuit interrupters upon the occurrence of overcurrent conditions; i.e. currents which are greater than the highest desirable operating current in an alternating current line. For example, the following references disclose various combinations of short-time, long-time, and instantaneous trip circuits: Boyd, U.S. Pat. No. 3,831,061; Pang et al, U.S. Pat. No. 4,442,472; Chen et al, U.S. Pat. No. 4,347,541; Howell, U.S. Pat. No. 4,297,741; and Shimp, U.S. Pat. No. 4,258,403. The Howell reference, U.S. Pat. No. 4,297,741, utilizes an iron core transformer for providing current signals to long-time and short-time trip circuits. An air core transformer is utilized to develop signals proportional to the rate of change of the current as an input to an instantaneous trip circuit. Additionally, the following references relate to various protection arrangements utilizing, for their operation, the rate of change of current (di/dt) or the rate of change of current in combination with one or more current levels: Keders et al, "A Current-Limiting Device for Service Voltages Up to 34.5 kV", July 18, 1976, 7 pages, IEEE PES Summer Meeting, Paper No. 76436-6; Bottger, "The Application of Is-Limiters in Three Phase Systems", pages 1-7, 1967, Calor-Emag Company, Germany; Calor-Emag Company, Leaflet No. 1197/6E, "Fault Levels Too High?", 2 pages, date unknown; M. C. Blythe, "Limiting Fault Currents Between Private and Public Networks", 4 pages Oct. 5, 1973, reprinted from Electrical Review, Calor-Emag; P. J. Kroon, "The Development and Application of a 69-kV Fault Current Limiter", pages 237-244, Apr. 1, 1971, 7th IEEE/PES Transmission and Distribution Conference and Exposition; "Pyristor Pyrotechnic Systems", Carbone Ferraz, Inc. brochure, date unknown; and "Prototype Fault Current Limiter", EPRI Report, EL-1396, May 1980, pages 7-1 to 7-19, 8-1, Palto Alto, Calif.
While these arrangements may be generally suitable for their intended purposes, prior protection arrangements operating in accordance with a time-current characteristic including inverse-time portions have only been capable of operation for overcurrents that have maximum current levels within the interrupting capabilities of the circuit interrupter as defined in terms of the instantaneous magnitude of the current at the time of interruption.